The present preferred embodiment relates to magnetic resonance imaging (MRI) systems, which employ a cryogenically cooled superconducting magnet and active gradient magnet coils. In particular, the preferred embodiment relates to such MRI systems in which the superconducting magnet is cooled by conduction cooling to a cryogenic refrigerator, or where a cooling loop system, or other low-cryogen-volume cooling system is in use. The embodiment may, however, be usefully applied also to superconducting magnets which are cooled by partial immersion in a bath of cryogen.
FIG. 1 shows a schematic axial part-cross-section through components of a cylindrical superconducting magnet in an MRI system suitable for improvement by application of the present preferred embodiment. The illustrated structure is essentially rotationally symmetrical about axis A-A. Annular superconducting coils 10 are thermally attached to a cooling loop pipe 12 through a crust layer 14. Each cooling loop pipe 14 includes a bore 16 arranged to carry liquid and/or gaseous cryogen in a circulating fashion to a cryogenic refrigerator, thereby maintaining coils 10 at an operating temperature below their superconducting transition temperature. In this example, the circulating cryogen is helium. A mechanical retaining structure 18 is provided, to support the magnet. As shown, this is typically also cooled by the cooling loop pipe. The superconducting magnet and other cooled components are enclosed within an outer vacuum chamber (OVC) 20, which is only partially represented in the drawing. The OVC and thermal radiation shields, together with associated components not illustrated, make up a cryostat for holding the magnet at a cryogenic temperature.
The OVC is at ambient temperature, and one or two thermal radiation shields 22, 24 may be provided between the magnet and the OVC, to intercept thermal radiation from the OVC before it reaches the magnet. In the illustrated example, two thermal radiation shields are provided. An outer thermal radiation shield 24 is cooled to a temperature of for example about 77K. This may be achieved by cooling by boiling of a sacrificial liquid nitrogen cryogen, or by operation of a mechanical refrigerator. Thermal radiation from the OVC, typically at a temperature of about 300K, is intercepted by this outer thermal radiation shield and is removed from the system by cooling of the outer thermal radiation shield. An inner thermal radiation shield 22 is cooled to about 4K by the cooling loop tube 12. In the illustrated arrangement, this is achieved by thermal conduction from the inner thermal radiation shield through shield support structure 26 and mechanical retaining structure 18.
As the magnet is itself cylindrical, a cylindrical bore 30, aligned to axis A-A extends through the magnet system, and allows access for a patient to be imaged. Typically, an imaging region is present at the axial mid-point of the bore, is essentially spherical and has a diameter of about 40-50 cm.
Within the bore of the OVC 20, a gradient coil assembly 32 is positioned. As is well known, such gradient coil assembly provides oscillating magnetic fields in orthogonal directions in the imaging region, as required in forming MRI images. These oscillating fields typically operate at a frequency from 1 Hz to 4 kHz.
In operation, the oscillating magnetic fields of the gradient coil assembly do not only extend into the imaging region, but stray magnetic fields from the gradient coil assembly reach the OVC.
The time-varying stray magnetic fields of the gradient coils 32 will induce appreciable Ohmic heating effects in both the thermal radiation shield(s) 22, 24 and in the superconducting coils 10 of the magnet system.
Typically, the stray time-varying magnetic fields from the gradient coil assembly will, directly or indirectly, induce eddy currents in metal parts of the cryostat, in particular metal bore tubes of the OVC 20 and the thermal shield(s) 22, 24. The structure of the OVC, if of metal, will provide shielding to the superconducting coils and the thermal radiation shield(s) 22, 24 from the stray magnetic fields from the gradient coil assembly 32. However, due to the static background magnetic field produced by the magnet coils 10, the eddy currents induced in the material of the OVC by the time-varying magnetic fields from the gradient coils produce Lorentz forces, resulting in mechanical vibrations of the OVC. These vibrations, which occur within the static magnetic field of the magnet coils 10, will in turn generate eddy currents in the material of the OVC which in turn produce secondary stray fields. These secondary stray fields can be much larger than the original stray fields produced by the gradient coils. The mechanical vibrations of the OVC will be particularly strong when a resonance vibration mode of the OVC bore tube is excited. Similarly, tertiary and further stray fields may be generated, effectively passing through the thermal radiation shield(s) 22, 24 to interact directly with the superconducting coils.
If resonant vibration modes and frequencies of the bore tubes of OVC 20 and thermal shield(s) 22, 24 are close together, as common in present magnet systems, the bore tubes behave as a chain of closely coupled oscillators, and resonance bands will occur. Eddy currents induced in the superconducting magnet coils from the time-varying magnetic field induced from the mechanically vibrating conducting surfaces as discussed above constitute a heat-load on the cooling system. This excess heat-load can result in a temperature rise of the coils, which can lead to a quench in the superconducting coils. Even with magnets which are cooled by partial immersion in a cryogen, the heat-load will cause an increase in cryogen consumption and/or an increase in power consumed by the cryogenic refrigerator.